In automotive evaporative emission systems, fuel vapor generated in the vehicle fuel tank is captured in a charcoal-filled canister and subsequently supplied to the engine air intake through a solenoid purge valve. Since the effectiveness of the system can be significantly impaired by faulty operation of a component or by a leak in one or more of the hoses or components, the engine controller is generally programmed to carry out a number of diagnostic algorithms for detecting such failures. If faulty operation is detected, the result is stored and a "check engine" lamp is activated to alert the driver so that corrective action can be taken.
Experience has shown that small leaks in the evaporative system can be particularly difficult to reliably detect and diagnose due to variability of fuel characteristics, driving schedules, and environmental conditions. While small leaks can theoretically be detected at engine idle by closing off the air vent, drawing the system below atmospheric pressure with engine vacuum, and then monitoring the change in system pressure, it is difficult to obtain reliable test data in a short period of time because of the time required to evacuate and establish vapor generation equilibrium in the system. Accordingly, what is needed is a method of quickly and reliably diagnosing evaporative emission system leaks at engine idle.